The substitution reaction at the step can be carried out in the absence of any solvent or in the presence of solvents inert to the reaction, for example aromatic hydrocarbons such as benzene, toluene and xylene; ethers such as diethyl ether, tetrahydrofuran (THF), and dioxane; halogenated hydrocarbons such as dichloromethane, 1,2-dichloroethane and chloroform; N,N-dimethylformamide (DMF); dimethyl